Newton’s Laws of Motion:
T HE L AW
OF I NERTIA
Force
. . . an action exerted upon a body,
in order to change its state,
either of rest or of moving uniformly . . .
Sir Isaac Newton
Newton’s First Law
(The Law of Inertia)
Every body perseveres in its state of rest
or of uniform motion in a straight line
unless it is compelled to change that state
by forces impressed upon it.
Systems in
Equilibrium
Which has greater net force?
You are driving your Porsche 911 Carrera along a straight testing track at a
constant speed of 150 km/h. You pass a 1971 Volkswagen Beetle
doing a constant
75 km/h. For which car is the net force greater?
Frame of Reference:
A coordinate system plus a time scale
formed by an observer
Inertial Frame of Reference:
A Frame of Reference with ZERO acceleration.
Is the Net Force Zero?
Is the Net Force Zero?
Is the Net Force Zero?
Is the Net Force Zero?
Is the Net Force Zero?
Is the Net Force Zero?
Newton’s First Law
normal force
normal force
friction force
zero net force
constant
velocity
pedal force
force of gravity
(weight)
Newton’s Laws of Motion:
T HE L AW
OF
A CCELERATION
The
FIRST LAW
states that
acceleration
is zero
if
net force
is zero.
Equilibrium of Forces
force of the hand

F  0
net force
is NOT
zero
force of gravity
(weight)
Object
accelerates!
What if net force is not zero?
An external net force causes an object to
accelerate.
There is either
a change in the
magnitude of the velocity
a change in the
direction of the velocity
or both.


F  ma


F  ma
Force = mass x acceleration
Acceleration is
proportional
to force . . .


F  ma
Force = mass x acceleration
Acceleration is
proportional
to force . . .
. . . and inversely
proportional to
the mass
Free Fall (again)


F  ma
 
Fw

F 

 a  g
m

F
m


F
m
Suppose you have a
speeding object with
velocity in +x direction.
In what direction must the
acceleration be in order for
the object to stop?
So in what direction must
you apply the force?

vtrain
normal force
friction force
engine force
weight of train

vtrain

a

FSuperman
applied force
NET FORCE

vtrain
magnitude
of velocity
decreases!
Fnet and
Acceleration
point in the
same
direction!
2ND Law of Motion Example
A worker applies a constant horizontal force with magnitude
20 N to a box with mass 40 kg resting on a level floor with
negligible friction. What is the acceleration of the box?
a = 0.5 m/s2
Units of Force, Mass, & Acceleration
Newton’s Laws of Motion:
THE LAW OF ACTIONREACTION
Forces Comes in Pairs
Action and Reaction (Force Pair)
Think of this...
If we lessen the support on the book
Then we feel that the book is lighter
Newton’s Third Law
(The Law of Action and Reaction)
For every action,
there is an equal
and opposite
reaction.
Newton’s Third Law
An Excellent
INQUIRY,
MY FRIEND!
Newton’s Third Law


FA on B   FB on A
The forces in
an action –
reaction pair
act on different
bodies!
Law of Action and
Reaction
Imagine that you are holding a book
weighing 4 N at rest on the palm of your
hand. Complete the following sentences:
a) A downward force of magnitude 4 N is
exerted on the book by ________.
the earth
b) An upward force of magnitude
________
________
4 N is exerted on the
book by
your hand.
c) Is the upward force in part (b) the
reaction to the downward force in part
NO
(a)?
d) The reaction to the force in part (a) is a force of
magnitude ________,
exerted on the
________
4N
earth by
________.
the
book Its direction is ________.
upward
e) The reaction to the force in part (b) is a force of
magnitude ________
exerted on your
________
4N
hand by
the
book Its direction is __________.
downward
________.
f) The forces in parts (a) and (b) are equal and opposite
first law.
because of Newton's ________
g) The forces in parts (b) and (e) are equal and opposite
third law.
because of Newton's ________
Now suppose that you exert an upward force of
magnitude 5 N on the book.
h) Does the book remain in equilibrium? NO
i) Is the force exerted on the book by your hand equal
and opposite to the force exerted on the book by the
earth? NO
j) Is the force exerted on the book by the earth equal
and opposite to the force exerted on the earth by the
book? YES
k) Is the force exerted on the book by your hand equal
and opposite to the force exerted on your hand by
the book? YES
Finally, suppose you snatch your hand away while the book is moving
upward.
l) How many forces then act on the book?
ONE
m) Is the book in equilibrium?
NO
weight of book
Review
A 12.0-kg object undergoes an acceleration
of 3.0 m/s2. What is the magnitude of the
resultant force acting on it? If this same
force is applied to a 9.0-kg object, what
acceleration is produced?
a)
b)
c)
d)
36 N; 4.0 m/s2
4.0 N; 36 m/s2
4.0 N; 4.0 m/s2
36 N; 36 m/s2
Review
A 12.0-kg object undergoes an acceleration
of 3.0 m/s2. What is the magnitude of the
resultant force acting on it? If this same
force is applied to a 9.0-kg object, what
acceleration is produced?
a)
b)
c)
d)
36 N; 4.0 m/s2
4.0 N; 36 m/s2
4.0 N; 4.0 m/s2
36 N; 36 m/s2
Review
Last August 6, 2012, NASA has successfully
landed the rover Curiosity on Martian soil.
Curiosity weighs about 8820 N here on Earth.
What should it weigh on Mars, where the
free-fall acceleration is 3.75 m/s2?
a)3375 N
b)33075 N
c)900 N
d)The weight would be
Review
Last August 6, 2012, NASA has successfully
landed the rover Curiosity on Martian soil.
Curiosity weighs about 8820 N here on Earth.
What should it weigh on Mars, where the
free-fall acceleration is 3.75 m/s2?
a)3375 N
b)33075 N
c)900 N
d)The weight would be